Semiconductor lasers are routinely used in the field of opto-electronics in such diverse areas as optical information processing, optical communication, laser processing, medical treatment and optical measurement. Although GaAs lasers with wavelengths of 780 nm-830 nm (i.e., near the infrared range) are most commonly used today, many applications in the optical industry require that the wavelength of coherent beams emitted by semiconductor lasers be even shorter. Research and development is being promoted to comply with this demand. By way of example, in the field of optical storage, shorter wavelengths are required to increase the storage density of an optical recording medium. For color display, blue is essential. In the field of laser printers, shorter wavelengths mean higher photon energy, which translates into making a relevant chemical reaction more efficient. Consequently, various attempts have been made to generate coherent blue light with wavelengths of 300 nm to 500 nm by combining a semiconductor laser that generates red or near infrared light with wavelengths of 600 nm to 1000 nm, and a wavelength converter that converts the fundamental light wave emitted from a semiconductor laser into light with shorter wavelengths. Such a wavelength converter typically utilizes a second-order optical nonlinearity (second-harmonic generation (or SHG), sum frequency generation, or difference frequency generation) in order to achieve an efficient conversion.
Examples of conventional wavelength converters include inorganic crystals such as KDP, ADP, and LiNbO.sub.3. However, such materials do not possess a sufficiently high nonlinearity. Furthermore, a relatively large crystal is necessary to perform wavelength conversion at high efficiency, with the result that it is difficult to reduce the size of the device.
Other examples of wavelength converters include a multi-quantum-well structure, as it is described by M. M. Fejer, S. J. B. Yoo, R. L. Byer, A Harwit, and J. S. Harris, Jr., Phys. Rev. Lett. 62, 1041 (1989), and by E. Rosenchef, P. Bois, J. Nagle, and S. DeLaitre, Electron. Lett., 25, 1063 (1989). Reported are the results of experiments in which a multi-quantum-well structure consisting of a GaAs layer and a GaAlAs layer disposed on the substrate is used for converting infrared light (with a wavelength of 10.6 .mu.m). These reports reveal that a multi-quantum-well structure (include the barrier layer and the well layer are made from a III-V compound) has a high nonlinearity. However, a problem exists in that a multi-quantum-well structure is not transparent to light for a wavelength shorter than infrared; it absorbs such a light, with the result that it cannot be used to shorten the wavelength of the coherent light emitted by the semiconductor laser. Moreover, transparency is required for visible light, including blue light, to generate blue light by a second-order optical effect.